1. Field of the Invention
The invention relates to the field of peak-limiting circuits, particularly for audio signals.
2. Prior Art
In numerous systems such as AM broadcasting, FM broadcasting, optical film recording by the "variable area" method, carrier telephony, disc recording, and others, peak levels of signals must be controlled. Clippers, in conjunction with other circuits are frequently employed for this purpose. The clipper-induced distortion must be carefully controlled since it can be perceived as offensive distortion. On the other hand, in some of these systems, particularly AM broadcasting, it is necessary to clip as much as possible to broadcast a "loud" signal.
In copending application, Ser. No. 816,955, filed July 19, 1977, entitled "Apparatus and Method For Peak-Limiting Audio Frequency Signals" and assigned to the assignee of this application, a circuit and method is described for peak-limiting audio signals. With this system the level of audible, clipper-induced distortion is controlled. the psychoacoustic phenomena of "masking" is used to maintain a maximum non-audible level of clipping (or a constant audible level of clipping). The actual amount of clipping varies depending upon the presence of signals needed to mask the clipper induced distortion. In general with this system the distortion caused by a clipper is determined by subtracting a clipper's output from its input. This distortion is compared in a plurality of critical bands with the input audio signal to provide a gain control signal for a variable gain stage. The output of this stage is coupled to a clipper.
In the presently described system a control signal is generated which may be employed with the above referenced system or other systems to establish a dynamic level of clipping based on whether the clipping is noticeable as opposed to whether the clipping is audible.